In the operation of extrusion machines, screws, and still more shear spindles, give rise to the problem that the material under pressure that is to be extruded, strives to escape from the cylinder not only through the outlet provided but also through the openings through which the drive ends of the screw or the shear spindle extend. On these grounds, the clearance in these places has been held as close as possible and the screw shaft or shear spindle shaft has been provided with flat screw-like grooves of the same screw direction as the screw, or the same screw direction as the shear spindle, in order for the extrusion material entering these clearances to be returned to the extruder or the shear spindle cylinder. These flat screw-like grooves are called "sealing threads" and are known through DE-PS 1529 801.
However, a screw thread of this kind is not sufficient in many applications. Hence, possibilities have been sought to increase the sealing action of these sealing threads through the introduction of sealing means. A form of the introduction of a sealing means which is known through prior public use consists in the arrangement of a special extruder, in which powder-form plastic material is plasticized and, in this plasticized fluid form, is pressed in through a channel to the sealing thread. This fluidized plastic material works as a lubricating and sealing means and prevents the rubber mixture, that is to be extruded, from escaping in an undesired manner along the drive end of the screw or of the shear spindle. This manner of sealing has, however, two very serious disadvantages. The expense for a special extruder, even though it is of small size, is considerable and influences the initial cost and maintenance cost of the extrusion machine considerably. The feeding of the thermic fluid plastic material results in a part of this fluid plastic material being mixed with the rubber mixture to be extruded, so that this rubber mixture now contains an ingredient which does not belong.
Through the process of DE-PS 32 12 159 this kind of sealing is improved in that a cooling and a warming is undertaken of that chamber in which the part of the shear spindle or screw provided with a sealing thread rotates. The annular clearance on the side turned away from the cylinder and toward the drive is cooled so that the viscosity of the injected fluid plastic material is here higher than in the part that turned toward the cylinder. The thermo plastic fluid plastic material pressed into the annular clearance hence tends to flow out of the sealing chamber toward the drive rather than into the cylinder serving as a working space for the mass to be extruded. The production technique and mechanical difficulties are not hereby abated, but rather increased, also the entrance of the sealing mass into the material to be extruded is not avoided, but favored.